Off-the-Record (OTR) Messaging allows you to have private conversations
over instant messaging by providing:

Encryption

No one else can read your instant messages.

Authentication

You are assured the correspondent is who you think it is.

Deniability

The messages you send do not have digital signatures that are
checkable by a third party. Anyone can forge messages after a
conversation to make them look like they came from you. However,
during a conversation, your correspondent is assured the messages
he sees are authentic and unmodified.

Perfect forward secrecy

If you lose control of your private keys, no previous conversation is compromised.

OTR library and toolkit

This is the portable OTR Messaging Library, as well as the toolkit to
help you forge messages. You need this library in order to use the other
OTR software on this page. [Note that some binary packages, particularly
Windows, do not have a separate library package, but just include the library
and toolkit in the packages below.] The current version is 4.1.0.

Java OTR library

This is the Java version of the OTR library. This is for developers of Java applications that want to add support for OTR. End users do not require this package. It's still early days, but you can download java-otr version 0.1.0 (sig).

OTR plugin for Pidgin

This is a plugin for Pidgin 2.x which implements
Off-the-Record Messaging over any IM network Pidgin supports.
The current version is 4.0.1.

OTR localhost AIM proxy

This software is no longer supported. Please use an IM client with native support for OTR.

This is a localhost proxy you can use with almost any AIM client in order
to participate in Off-the-Record conversations. The current version is
0.3.1, which means it's still a long way from done. Read the README
file carefully. Some things it's still missing:

Please see our OTR-enabled software page.
The OTR functionality is separated into the
Off-the-Record Messaging Library (libotr), which is an LGPL-licensed
library that can be used to (hopefully) easily produce OTR plugins for
other IM software, or for other applications entirely.

The pidgin-encryption plugin provides encryption and authentication,
but not deniability or perfect forward secrecy. If an attacker or a
virus gets access to your machine, all of your past
pidgin-encryption conversations are retroactively compromised. Further,
since all of the messages are digitally signed, there is
difficult-to-deny proof that you said what you did: not what we want
for a supposedly private conversation!

How is this different from Trillian's SecureIM?

SecureIM doesn't provide any kind of authentication at all! You
really have no idea (in any kind of secure way) to whom you're speaking,
or if there is a "man in the middle" reading all of your messages.

How is this different from SILC?

SILC uses a completely separate network of servers and underlying
network protocol. In some environments, such as firewalled or corporate
setups, where a local proprietary IM protocol may be in use, SILC may
not be available. Further, in its normal mode of operation, all SILC messages
are shared with the SILC servers; if you want to send messages that can only
be read by the person with whom you're communicating, you need to either
(1) arrange a pre-shared secret in advance (which hampers perfect forward
secrecy), or (2) be able to do a direct peer-to-peer connection to the other
person's client, in order to do a key agreement (which may not be possible in
a NAT or firewall situation).